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Dissociable Prior Influences of Signal Probability and Relevance on Visual Contrast Sensitivity

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Dissociable Prior Influences of Signal Probability and Relevance on Visual Contrast Sensitivity Correction PSYCHOLOGICAL AND COGNITIVE SCIENCES Correction for “Dissociable prior influences of signal probability and relevance on visual contrast sensitivity,” by Valentin Wyart, Anna Christina Nobre, and Christopher Summerfield, which appeared in issue 9, February 28, 2012, of Proc Natl Acad Sci USA (109:3593–3598; first published February 13, 2012; 10.1073/ pnas.1120118109). The authors note that on page 3597, right column, the third equation (for soft threshold nonlinearity) appeared incorrectly. The corrected equation appears below: x Γ½x ¼ x þ α − exp α www.pnas.org/cgi/doi/10.1073/pnas.1204601109 6354 | PNAS | April 17, 2012 | vol. 109 | no. 16 www.pnas.org Downloaded by guest on September 30, 2021 Dissociable prior influences of signal probability and relevance on visual contrast sensitivity Valentin Wyarta,1, Anna Christina Nobrea,b, and Christopher Summerfielda aDepartment of Experimental Psychology, University of Oxford, Oxford OX1 3UD, United Kingdom; and bOxford Centre for Human Brain Activity, Department of Psychiatry, University of Oxford, Warneford Hospital, Oxford OX3 7JX, United Kingdom Edited by Ranulfo Romo, Universidad Nacional Autonoma de Mexico, Mexico City, DF, Mexico, and approved January 18, 2012 (received for review December 7, 2011) According to signal detection theoretical analyses, visual signals increasing the baseline activity of signal-selective units or by occurring at a cued location are detected more accurately, whereas shifting the observer’s decision criterion toward one of the two frequently occurring ones are reported more often but are not better responses. However, the binary classification of stimuli as signal- − distinguished from noise. However, conventional analyses that present (S+) and signal-absent (S ) used by conventional SDT estimate sensitivity and bias by comparing true- and false-positive analyses makes it difficult to arbitrate between these different rates offer limited insights into the mechanisms responsible for these possibilities, for a number of reasons. First, the conventional ap- effects. Here, we reassessed the prior influences of signal probability proach does not distinguish between the two successive sources and relevance on visual contrast detection using a reverse-correla- of performance-limiting noise in the decision process: external tion technique that quantifies how signal-like fluctuations in noise noise (physical noise in the stimulus itself) and internal noise predict trial-to-trial variability in choice discarded by conventional (physiological noise during stimulus processing). This conflation analyses. This approach allowed us to estimate separately the makes it hard to pinpoint the locus of contextual influences on sensitivity of true and false positives to parametric changes in signal signal detection (e.g., whether effects occur upstream or down- energy. We found that signal probability and relevance both stream from internal noise) (16). Second, the binary classifica- increased energy sensitivity, but in dissociable ways. Cues predicting tion of stimuli does not allow to measure sensitivity separately in − the relevant location increased primarily the sensitivity of true the S+ and S categories. This is important, because different positives by suppressing internal noise during signal processing, mechanisms make distinct predictions as to whether their effects COGNITIVE SCIENCES whereas cues predicting greater signal probability increased both on sensitivity should grow or shrink with signal strength (17). PSYCHOLOGICAL AND the frequency and the sensitivity of false positives by biasing the Reverse-correlation analyses offer a powerful complement to baseline activity of signal-selective units. We interpret these findings conventional analyses, by permitting the measurement of ob- in light of “predictive-coding” models of perception, which propose server sensitivity to small, noise-driven changes in image statistics separable top-down influences of expectation (probability driven) (18, 19). Here we adopted a reverse-correlation approach to and attention (relevance driven) on bottom-up sensory processing. identify the mechanisms by which signal probability and rele- vance influence signal detection. To do so, we quantified the decision making | psychophysics | visual perception amount of signal energy present in each noisy stimulus by con- volution with a pool of visual filters that approximate the re- ignal detection theory (SDT) proposes that sensitivity can be ceptive fields of orientation-selective neurons in early visual Scalculated by comparing true- and false-positive rates (1, 2). In cortex (20, 21). This parametric characterization of external a typical detection task, subjects are asked to judge whether a noisy noise allowed us to estimate the sensitivity of human observers to + − stimulus does or does not contain a low-energy signal, allowing signal-like fluctuations separately in S and S stimuli. In con- researchers to classify stimuli judged as containing the signal into junction with a signal detection task in which two types of cues true and false positives (“hits” and “false alarms,” respectively), provided mutually independent information about probability and stimuli judged as not containing the signal into true and false and relevance, this approach allowed us to dissociate and arbi- negatives (“correct rejections” and “misses”). SDT posits that trate between their candidate mechanisms. d′ performance can be summarized by two statistics: , indexing Results sensitivity to signal occurrence in signal-to-noise units, and c, Probability × Relevance Cueing Procedure. fi reflecting a bias to report signal occurrence (modeled as a decision While xating centrally, subjects viewed two simultaneously presented stimuli in colored criterion). Using this approach, it is now well established that cues fi A that predict the location of a behaviorally relevant signal increase placeholders located in their left and right visual elds (Fig. 1 ). Their task was to report whether a signal was present (S+)or sensitivity (3–6) by improving the precision of visual processing (7– − absent (S ) in one of the two placeholders, indicated by a color- 10). By contrast, cues that predict a greater probability of signal matched probe presented after stimulus offset. The target signal occurrence alone are believed to have no influence on sensitivity was a vertical Gabor pattern of two cycles per degree of visual (11, 12) but instead bias observers to report signal occurrence by angle, presented at a fixed contrast titrated for each subject be- adopting a more liberal decision criterion. fore the experiment. All stimuli were embedded in visual noise Over the past 50 years, SDT has provided a versatile de- whose frequency characteristics closely matched those of the scription of decision processes, both in laboratory experiments signal (Methods). We manipulated signal probability at the block and in real-world situations such as medical diagnostics, by dis- sociating between an observer’s sensitivity and bias, two quanti- ties that had traditionally been difficult to tease apart (13). How- Author contributions: V.W., A.C.N., and C.S. designed research; V.W. performed research; ever, SDT has remained largely silent about the computational V.W. contributed new reagents/analytic tools; V.W. and C.S. analyzed data; and V.W., mechanisms by which sensitivity and bias are influenced by con- A.C.N., and C.S. wrote the paper. textual information such as the prior probability and relevance of The authors declare no conflict of interest. the signal. Indeed, changes in signal-to-noise sensitivity can occur This article is a PNAS Direct Submission. either by amplifying the responses of signal-selective units (14) or 1To whom correspondence should be addressed. E-mail: [email protected]. by suppressing performance-limiting noise without amplifying the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. signal per se (15). Similarly, changes in bias can arise either by 1073/pnas.1120118109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1120118109 PNAS Early Edition | 1of6 Fig. 1. Task structure and conventional signal detection analyses. (A) Task structure. Subjects viewed two simultaneously presented stimuli in colored placeholders located in their left and right visual fields. Their task was to report whether a signal was present or absent in one of the two placeholders, indicated by a color-matched probe. We manipulated signal probability at the block level using a cue indicating the prior probability of signal occurrence in each of the two colored placeholders, and signal relevance at the trial level using a prestimulus cue indicating the most likely color of the poststimulus probe. (B) Signal probability biases detection judgments by increasing hit and false-alarm rates to a similar extent. (C) Signal relevance improves the precision of signal processing by selectively decreasing the false-alarm rate for cued stimuli. *P < 0.05; **P < 0.01; ns, nonsignificant effect. Error bars indicate SEM. − level using a cue indicating the prior probability of signal oc- within S+ and S stimulus categories using binomial parametric currence in each of the two colored placeholders (0.67/0.33, 0.50/ regression (Methods). The regressed energy sensitivity provides an 0.50, or 0.33/0.67), and signal relevance at the trial level using estimate of the strength of the relationship between the amount a prestimulus cue indicating the most likely color of the post- of
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